1. Field of the Invention
The present invention relates to a planar light waveguide that maintains a single mode of an incident laser light source for obtaining a small spot size with a short wavelength laser and to a planar focusing grating coupler that uses a fine diffraction grating for focusing with the high numerical aperture.
2. Discussion of Related Art
A pickup head, which is an integral device of an optical disk drive well known as a CD, DVD and BD, comprises optical parts such as an optical lens and an optical isolator, and a lot of other parts such as a laser diode, a photodiode, and a focusing/tracking servo device, so that what is worst is that it is bulkier and heavier than a magnetic disk drive head known as a hard disk.
Accordingly, it serves as a drawback in terms of an access rate and a data transfer rate, etc. Recently, an ultra small type optical disk drive is researched and developed as a mass storage for applying to mobile/portable information appliances, and a technology is required that allows the foregoing drawbacks to be drastically improved.
One such method suggested is to use a focusing grating coupler with a fine optical grating rather than a substance lens, which allows significantly reducing a size and lowering a weight. The focusing grating coupler is a device fabricated on a plane, with an ultra fine grating that is finer than a light wavelength rather than with a spherical lens made of glass and plastic, and focuses incident light onto one point, with which the integral portion of the pickup head can be reduced below a semiconductor chip size, and accordingly, the access rate and the data transfer rate can also be significantly improved.
In this case, in order to increase record density while replacing the substance lens with the planar focusing grating coupler, the minimum spot size of the laser light should be sufficiently small, in which the spot size has a relation as shown in the following equation (1).
                              spot          ⁢                                          ⁢          size                =                  0.61          ⁢                      λ                          n              ·              NA                                                          (        1        )            where λ indicates incident light, n indicates a refractive index of a medium, NA indicates a numerical aperture of a lens and is shown as n sinθ in terms of an aperture angle θ.
Therefore, in order to achieve the minimum spot size, generally, a shorter wavelength laser as an incident light source, and/or the higher numerical aperture (NA) of the lens should be used. For this, a short focal length and a large lens size, that is, a coupling area of the grating, should be increased or a coupling angle should be increased, in order to increase the numerical aperture NA while using laser with a short wavelength as possible.